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Static and Kinetic Friction OBJECTIVES: 1. To determine the factors affecting kinetic friction. 2. To compare forces of static and kinetic friction. 3. To calculate the coefficient of friction between surfaces. INTRODUCTION: Friction is a force that resists motion between an object and its surroundings. Whenever an object interacts with its surroundings, friction is unavoidable. In this experiment, we will consider only frictional forces between two flat surfaces that are in contact with each other. Frictional forces between two surfaces are always parallel to the surfaces and in the opposite direction of motion. Friction that prevents an object from moving is called static friction, and friction that resists motion that’s already occurring is called kinetic friction. What determines the magnitude of these frictional forces between two surfaces? In this experiment, you will investigate the variables that affect friction and determine a way to predict the frictional force between two surfaces. You’ll also determine how the magnitudes of the forces of static and kinetic friction compare to each other. METHOD: Part I: Nature of Surfaces 1. Answer interpretation question #1 before beginning the procedure. 2. Navigate to the honors physics shared files folder (from either the school web site physics page or the Honors Physics I moodle page). Open the “Class Materials,” “LoggerPro” folders. Click on the file “frictionlab” and open the file. 3. Connect the dual-range force sensor to channel 1 of the LabPro interface. Make sure the switch is set to the “50 N” range. 4. Connect the force sensor to the friction block as shown below (felt side facing down). You may need to use a paper clip to make the connection. Document1 5. Make sure that the force sensor is not pulling on the block. Then zero the sensor by clicking on the “Set Zero Point” button in the LoggerPro toolbar. The displayed force reading should now be close to 0.00. 6. Place a brick on top of the friction block. Make sure the block is located at a spot on the table where it can be pulled for at least a meter without running off the table’s edge. 7. Click on “Collect” in LoggerPro while slowly dragging the block across the table at constant speed. Make sure to pull horizontally on the force sensor (not at an upward angle). Continue pulling until a constant, horizontal plot of force vs. time is displayed on your graph. 8. Click and drag over the region of your graph where the force remains fairly constant. Then click on the “Statistics” button in the LoggerPro toolbar. Note the mean value for the frictional force between the felt and table and record it in your data table. 9. Click on “Store Latest Run” in the “Experiment” menu. Place two bricks end to end on the table. Then place the friction block wood side down on top of the bricks and connect it to the force sensor, as shown below. 10. Place a brick on top of the friction block and repeat step 6 (zero the force sensor again if necessary. 11. Repeat step 7 (choose “Latest” when prompted). Note the mean value for the frictional force between the wood and brick surfaces and record it in your data table. 12. Click on “Store Latest Run” in the “Experiment” menu. This time, place the friction block felt side down on top of the bricks. Place a brick on top of the friction block, zero the force sensor if necessary, and repeat steps 6 and 7 (choose “Latest” when prompted). Note the mean value for the frictional force between the felt and brick surfaces and record it in your data table. 13. Print a copy of your graph (make sure the statistics boxes don’t cover the plotted curves) for each lab partner and answer interpretation question #2. Document1 Part II: Normal Force 1. Select “Clear All Data” from the “Data” menu and close the statistics box. You will now investigate how the force pressing the two surfaces together affects friction. This force is called the normal force, and it represents the surface pushing back on the object. Its direction is always perpendicular to the surfaces. 2. Connect the force sensor to the friction block (felt side facing down on the lab table). Zero the sensor if necessary and place a brick on top of the block. Click on “Collect” in LoggerPro while slowly dragging the block across the table at constant speed. Make sure to pull horizontally on the force sensor (not at an upward angle). Continue pulling until a constant, horizontal plot of force vs. time is displayed on your graph. 3. Click and drag over the region of your graph where the force remains fairly constant. Then click on the “Statistics” button in the LoggerPro toolbar. Note the mean value for the frictional force between the felt and table and record it in your data table. 4. Click on “Store Latest Run” in the “Experiment” menu. Then add a second brick to the top of the friction block and repeat steps 2 and 3. Repeat for three and four bricks. Print a copy of your graph (make sure the statistics boxes don’t cover the plotted curves) for each lab partner. Then answer interpretation question #3. Part III: Surface Area 1. Select “Clear All Data” from the “Data” menu and close the statistics box. You will now investigate how an object’s surface area affects friction. 2. Connect the force sensor to the friction block (felt side facing down on the lab table, as before). Zero the sensor if necessary and place a single brick on top of the block. Click on “Collect” in LoggerPro while slowly dragging the block across the table at constant speed. Make sure to pull horizontally on the force sensor (not at an upward angle). Continue pulling until a constant, horizontal plot of force vs. time is displayed on your graph. 3. Click and drag over the region of your graph where the force remains fairly constant. Then click on the “Statistics” button in the LoggerPro toolbar. Note the mean value for the frictional force between the large surface of the felt and the table and record it in your data table. Document1 4. Click on “Store Latest Run” in the “Experiment” menu. Then turn the friction block onto its side so that the small felt surface is touching the table (as shown below). Repeat steps 2 and 3. Answer interpretation question #4. Part IV: Velocity 1. Select “Clear All Data” from the “Data” menu and close the statistics box. You will now investigate how the speed at which an object is moving affects friction. 2. Connect the force sensor to the friction block (felt side facing down on the lab table, as before). Zero the sensor if necessary and place a single brick on top of the block. Click on “Collect” in LoggerPro while slowly dragging the block across the table at constant speed. Make sure to pull horizontally on the force sensor (not at an upward angle). Continue pulling until a constant, horizontal plot of force vs. time is displayed on your graph. 3. Click and drag over the region of your graph where the force remains fairly constant. Then click on the “Statistics” button in the LoggerPro toolbar. Note the mean value for the frictional force between the felt and the table and record it in your data table. 4. Click on “Store Latest Run” in the “Experiment” menu. Repeat steps 2 and 3 while dragging the block at a faster constant speed. Be sure to analyze only the portion of the graph where the block was moving at constant speed. Then answer interpretation questions 5-6. Part V: Static vs. Kinetic Friction 1. Select “Clear All Data” from the “Data” menu and close the statistics box. You will now investigate how static friction compares to kinetic. Static friction is the force that prevents the block from initially moving whereas kinetic friction is the force that opposes it while it’s moving. 2. Place two bricks end to end on the table. Then place the friction block felt side down on top of the bricks and connect it to the force sensor. Place a brick on top of the friction block and zero the force sensor again if necessary. 3. Click on “Collect” in LoggerPro while very slowly increasing the applied force until the block just begins to move. Make sure to pull horizontally on the force sensor (not at an upward angle). Continue pulling until a constant, horizontal plot Document1 of force vs. time is displayed on your graph. If the edge of the block gets caught in the crack between the bricks, try starting with the front edge of the block just beyond the crack. 4. Click and drag over the region of the graph that was plotted before the block started to move. Then click on the “Statistics” button in the LoggerPro toolbar. Note the maximum value for the frictional force between the felt and the table. This was the value of the frictional force just before the block began to move (static friction). Record this in your data table. 5. Click and drag over the region of your graph where the block was moving at a constant speed. Then click on the “Statistics” button in the LoggerPro toolbar. Note the mean value for the frictional force between the felt and the table. This was the frictional force after the block began to move (kinetic friction). Record this in your data table. 6. Print a copy of your graph (make sure the statistics boxes don’t cover the plotted curves) for each lab partner. Then answer interpretation questions 7-8. Part VI: Coefficients of Friction 1. The coefficient of friction between two surfaces, is defined as the ratio between 𝑓 the frictional force, f, and the normal force, N, or 𝜇 = . Since the table was 𝑁 pushing back with enough force to keep the block in equilibrium, the normal force was equal to the weight of the block plus brick(s). 2. Use the high capacity balance to measure the mass of your block and a single brick. Record your results in the data table. Assuming that all of the bricks have the same mass, determine the normal force for each of the scenarios in Part II and complete that column of the data table. Don’t forget to compute the weight of each mass combination. 3. Using the measured frictional forces from Part II, compute the coefficient of friction for each mass combination and complete that column of the data table. 4. Using the measured frictional forces from Part I, compute the coefficient of friction for each of the surface combinations. Recall that a single brick was used along with the friction block in Part I. Then answer interpretation questions 9-10. 5. When police investigate an accident scene, one of the things they need to know is the coefficient of friction between the rubber tires and the road surface. To determine this, they use a device called a drag sled. The drag sled consists of a block with attached rubber tire tread that can be dragged across the road surface. Document1 Measure the mass of the drag sled on the high capacity balance and enter it in the data table. 6. Place the drag sled on the lab table (tire tread facing the table) and place a brick on top of it. Select “Clear All Data” from the “Data” menu and close the statistics box. Click on “Collect” in LoggerPro while slowly dragging the drag sled across the table at constant speed. Make sure to pull horizontally on the force sensor (not at an upward angle). Continue pulling until a constant, horizontal plot of force vs. time is displayed on your graph. 7. Click and drag over the region of your graph where the force remains fairly constant. Then click on the “Statistics” button in the LoggerPro toolbar. Note the mean value for the frictional force between the drag sled and the table and record it in your data table. 8. Calculate the coefficient of kinetic friction between the drag sled and the lab table and enter it in the data table. Finally, answer interpretation questions 11-14. Document1 DATA: Part I: Nature of Surfaces Surfaces Average Frictional Force (N) Felt on table Wood on brick Felt on brick Part II: Normal Force Number of Bricks Average Frictional Force (N) 1 2 3 4 Document1 Part III: Surface Area Area Average Frictional Force (N) Large Felt Small Felt Part IV: Velocity Speed Average Frictional Force (N) Low High Part V: Static vs. Kinetic Friction Type of Friction Static Frictional Force (N) Kinetic Document1 Part VI: Coefficients of Friction Mass of Friction Block: _______________ g Mass of Brick: _______________ g Number of Bricks Normal Force (N) Frictional Force (N) Coefficient of Kinetic Friction Normal Force (N) Frictional Force (N) Coefficient of Kinetic Friction 1 2 3 4 Surfaces Felt on table Wood on brick Felt on brick Mass of Empty Drag Sled: _______________ g Mass of Drag Sled with Brick: _______________ g Normal Force: _______________ N Coefficient of Friction: _______________ Document1 DATA TREATMENT: Normal force in Part VI Coefficient of friction in Part VI Be sure to include graphs for Parts I, II, V, and interpretation question #2. INTERPRETATIONS: 1. What are some factors that you think would affect the magnitude of the kinetic frictional force between two surfaces? 2. Does frictional force seem to depend upon the nature of the surfaces that are in contact with each other? What might account for this? 3. Plot a graph of average frictional force vs. number of bricks (you’ll want to start a second copy of LoggerPro in order to do this). How does frictional force appear to be related to normal force? Based upon your graph (and the equation for the best-fit line), predict the frictional force if 10 bricks were to be stacked on the friction block. 4. How did the frictional force for the larger surface compare to the smaller for each material? Based upon your data, does friction seem to vary appreciably with surface area as long as the surface and normal force remain constant? What might account for this? 5. How did the frictional force for the lower speed compare to the higher speed? Based upon your data, does friction seem to vary appreciably with velocity as long as the surface and normal force remain constant? 6. Based upon everything you’ve done so far, upon what factors does the frictional force between two surfaces appear to depend? 7. How does the magnitude of static friction appear to compare to kinetic friction? Suppose you wish to slide your living room couch across the floor. Based upon this experiment, would it be more difficult to initially set the couch into motion or to keep it moving? 8. Did the force of static friction remain constant from when you first pulled on the block up until the time it started to move? As you began to pull harder and harder, explain what the static frictional force was doing up until the point the block began to move. 9. Based upon your experimental results, does coefficient of friction appear to vary Document1 appreciably with normal force when the surfaces are held constant? 10. Based upon your experimental results, does coefficient of friction appear to vary appreciably for different surfaces when the normal force is held constant? 11. Suppose a 2000 kg car whose tires are have the same composition as the drag sled is driving on a level surface who frictional characteristics are similar to that of the lab table. The driver loses control of the car while going around a turn and skids to a stop. Draw a free-body diagram showing all forces acting on the car and use Newton’s second law of motion to determine its acceleration while skidding. Show all of your work! 12. If the police measure the length of the skid marks to be 175 m, how fast was the driver going when he started to skid? Show all of your work! Express your answer in both m/s and mph. 13. If the posted speed limit on the road was 45 mph, could the driver be charged with exceeding the posted speed limit? 14. Based upon the results of this experiment, evaluate your answer to interpretation question #1. How would you answer this question knowing what you know now? ANALYSIS OF ERROR: 1. Suppose you had accelerated the friction block or drag sled (in the direction of motion) instead of pulling it at constant velocity. Would this have made your measured frictional forces too large or too small? Use Newton’s second law to explain your answer. 2. How would the error in #1 have affected your computed coefficients of friction? 3. If you were to do Part I of this experiment over again, would you get the exact same results for your computed coefficients of friction for the surfaces? What could account for any differences? Document1